FIG. 1 is a block diagram which schematically illustrates a known arrangement for monitoring the temperature at a plurality of temperature sensors T1-TN. Typically, this type of arrangement is incorporated within a control system for controlling the heating, air conditioning and ventilating of a building, wherein the temperature values obtained from temperature sensors T1-TN are used as feedback for the control system. In the case of many control systems, the circuitry of the control system, including the arrangement for monitoring, is subjected to severe temperature fluctuations which substantially alter the performance of the circuitry. For example, the circuitry of a control system may be located within a machinery room located upon the roof of a building wherein the temperature may fluctuate from below 0.degree. F. to above 130.degree. F.
The arrangement of FIG. 1 includes a circuit board 100, a plurality of resistive thermal devices 102 for sensing various ambient temperatures, a source 104 for providing current to devices 102, a signal conditioning circuit 106 associated with each of devices 102, a multiplexor 108, an analog-to-digital converter 110 and a microprocessor 112. Devices 102 are illustrated as being external to board 100 to represent a typical application wherein devices 102 may be tens or hundreds of feet from board 100.
In operation, microprocessor 112 is programmed to periodically sample the voltage at device 102 and voltage source 104 by variously selecting the appropriate channels of multiplexor 108. Various conditioning functions, such as filtering, are performed on the signals from device 102 and voltage source 104, and the signals are converted from an analog to a digital signal by convertor 110. Under ideal conditions--such as constant and optimum circuit board temperature, identical circuit characteristics from channel to channel, and circuitry which does not have changes is characteristics which vary with time--microprocessor 112 will sample the same digital signal from devices 102 when all of devices 102 are at the same temperature. Additionally, under ideal conditions, microprocessor 112 will sample the same digital signal from a given device 102, for a given temperature, regardless of the age and/or temperature of board 100.
Unfortunately, ideal conditions are not typically available, and the provision of ideal conditions is so costly that it is not practical or economically feasible to provide these conditions. Accordingly, various schemes have been developed to deal with the problems caused by variations in circuit characteristics due to fluctuations in circuit temperatures and differences in circuit characteristics from channel to channel.
In one scheme, the voltage at source 104 is monitored to compensate for changes in the temperature of board 100 since the voltage at source 104 is dependent upon the temperature of board 100. Based upon the voltage at source 104 of the board 100, microprocessor 112 is programmed to add or subtract an offset value from each of the digital signals representative of the temperatures sensed at one of devices 102. One problem with this scheme is that the digital value representative of the voltage at source 104 sampled by microprocessor 112 will also vary depending upon the age and/or temperature of circuit board 100. Accordingly, using such an arrangement does not provide an accurate reference for the purpose of correcting errors introduced in the digital signals representative of the temperatures sensed at one of devices 102, wherein the errors are caused by changes in circuit characteristics caused be aging and/or temperature fluctuations. In addition, the circuitry utilized in an arrangement such as that illustrated in FIG. 1, is not linear and the mere provision of offset values will not typically provide adequate error compensation. More specifically, the error introduced in the digital signals representative of the temperatures sensed at one of devices 102 is not only a function of changes in the temperature of the circuitry on board 100, but also a function of the temperature sensed at one of devices 102. Thus, merely compensating for changes in the voltage at source 104 does not take into account the non-linearity of the circuitry of board 100.
In a second scheme, directed to compensating for the differences in circuit characteristics from channel to channel, microprocessor 112 is programmed to add or subtract a particular offset value from each of the digital signals representative of the temperatures sensed at devices 102 depending upon which device 102 (T1-TN) is being sampled. The offset values are normally determined by a technician during the process of calibrating the circuitry. More specifically, a reference temperature is applied to each device 102 while board 100 is at ambient temperature from which the technician determines an offset value to be associated with each channel. These offset values serve to adjust the digital values representative of the temperature provided each channel. Subsequently, the offset values are programmed into the microprocessor software such that for a given reference temperature, the microprocessor 112 will produce similar temperature values regardless of the channel.
As with the first scheme discussed above, certain problems are inherent in the second scheme due to the non-linear characteristics of board 100 circuitry. More specifically, the channel-to-channel discrepancies are dependant upon both the particular temperature sensed by device 102 associated with the channel and the temperature of board 100 circuitry. In addition, the offset values are only determined for a single reference temperature and a single board 100 temperature. Accordingly, the set of offset values determined by the technician will only provide accurate discrepancy correction when one of devices 102 is sensing the reference temperature and board 100 is at the same temperature that it was at during calibration. Moreover, an accurate discrepancy correction for this single combination of reference and board temperature requires that the technician accurately determined the offset values.
Consequently, there remains the need for a system including an arrangement for compensating for errors introduced by the circuitry used for determining values representative of time-dependent variables such as temperature. These errors are of the type which can be attributed to the effects of the temperature changes in system circuitry, the differences in the particular circuitry associated with each of a plurality of variables, and the aging of system circuitry.